Alumina layer with controlled texture

ABSTRACT

A new and refined method to produce α-Al 2 O 3  layers in a temperature range of from about 750 to about 1000° C. with a controlled growth texture and substantially enhanced wear resistance and toughness than the prior art is disclosed. The α-Al 2 O 3  layer of the present invention is formed on a bonding layer of (Ti,Al)(C,O,N) with increasing aluminium content towards the outer surface. Nucleation of α-Al 2 O 3  is obtained through a nucleation step being composed of short pulses and purges consisting of Ti/Al-containing pulses and oxidising pulses. The α-Al 2 O 3  layer according to the present invention has a thickness ranging from about 1 to about 20 μm and is composed of columnar grains. The length/width ratio of the alumina grains is from about 2 to about 12, preferably from about 4 to about 8. The layer is characterized by a strong (104) growth texture, measured using XRD, and by low intensity of (012), (110), (113), (024) and (116) diffraction peaks.

RELATED APPLICATION DATA

This application is a divisional application of U.S. application Ser. No. 11/265,797, filed Nov. 2, 2005, which claims priority under 35 U.S.C. §119 and/or §365 to Swedish Application No. 0402693-6, filed Nov. 5, 2004, the entire contents of each of these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a coated cutting tool insert designed to be used in metal machining. The substrate is a cemented carbide, cermet, ceramics or cBN on which a hard and wear resistant coating is deposited. The coating exhibits an excellent adhesion to the substrate covering all functional parts thereof. The said coating is composed of one or more refractory layers of which at least one layer is a strongly textured alpha-alumina (α-Al₂O₃) deposited in the temperature range of from about 750 to about 1000° C.

A crucial step in the deposition of different Al₂O₃ polymorphs is the nucleation step. κ-Al₂O₃ can be grown in a controlled way on {111} surfaces of TiN, Ti(C,N) or TiC having the fcc structure. TEM has confirmed the growth mode which is that of the close-packed (001) planes of κ-Al₂O₃ on the close-packed {111} planes of the cubic phase with the following epitaxial orientation relationships: (001)_(κ)//(111)_(TiX); [100]_(κ)//[112]_(TiX). An explanation and a model for the CVD growth of metastable κ-Al₂O₃ have proposed earlier (Y. Yoursdshahyan, C. Ruberto, M. Halvarsson, V. Langer, S. Ruppi, U. Rolander and B. I. Lundqvist, Theoretical Structure Determination of a Complex Material: κ-Al₂O₃, J. Am. Ceram. Soc. 82(6)(1999)1365-1380).

When properly nucleated, κ-Al₂O₃ layers can be grown to a considerable thickness (greater than about 10 μm). The growth of even thicker layers of κ-Al₂O₃ can be ensured through re-nucleation on thin layers of, for example TiN, inserted in the growing κ-Al₂O₃ layer. When nucleation is ensured, the κ→α transformation can be avoided during deposition by using a relatively low deposition temperature (less than about 1000° C.). During metal cutting, the κ→α a phase transformation has confirmed to occur resulting in flaking of the coating. In addition, there are several other reasons why α-Al₂O₃ should be preferred in many metal cutting applications. As shown earlier α-Al₂O₃ exhibits better wear properties in cast iron (U.S. Pat. No. 5,137,774).

However, the stable α-Al₂O₃ phase has been found to be more difficult to be nucleated and grown at reasonable CVD temperatures than the metastable κ-Al₂O₃. It has been experimentally confirmed that α-Al₂O₃ can be nucleated, for example, on Ti₂O₃ surfaces, bonding layers of (Ti,Al)(C,O) or by controlling the oxidation potential using CO/CO₂ mixtures as shown in U.S. Pat. No. 5,654,035. The bottom line in all these approaches is that nucleation must not take place on the 111-surfaces of TiC, TiN, Ti(C,N) or Ti(C,O,N), otherwise κ-Al₂O₃ is obtained.

It should also be noted that in the prior-art methods higher deposition temperatures (about 1000° C.) are usually used to deposit α-Al₂O₃. When the nucleation control is not complete, as is the case in many prior-art products, the produced α-Al₂O₃ layers have, at least partly, been formed as a result of the κ-Al₂O₃→α-Al₂O₃ phase transformation. This is especially the case when thick Al₂O₃ layers are considered. These kinds of α-Al₂O₃ layers are composed of larger grains with transformation cracks. These layers exhibit much lower mechanical strength and ductility than the α-Al₂O₃ layers that are composed of nucleated α-Al₂O₃. Consequently, there is a need to develop techniques to control the nucleation step of α-Al₂O₃.

The control of the α-Al₂O₃ polymorph in industrial scale was achieved in the beginning of the 1990's with commercial products based on U.S. Pat. No. 5,137,774. Later modifications of this patent have been used to deposit α-Al₂O₃ with preferred coating textures. In U.S. Pat. No. 5,654,035, an alumina layer textured in the (012) direction and in U.S. Pat. No. 5,980,988 in the (110) direction are disclosed. In U.S. Pat. No. 5,863,640, a preferred growth either along (012), or (104) or (110) is disclosed. U.S. Pat. No. 6,333,103 describes a modified method to control the nucleation and growth of α-Al₂O₃ along the (10(10)) direction. US20020155325A1 describes a method to obtain a strong (300) texture in α-Al₂O₃ using a texture modifying agent (ZrCl₄) The processes discussed above use all high deposition temperatures of about 1000° C.

US 2004/0028951A1 describes a new state-of-the-art technique to achieve a pronounced (012) texture. The commercial success of this kind of product demonstrates the importance to refine the CVD process of α-Al₂O₃ towards fully controlled textures.

It is well established that the water gas shift reaction, in the absence of H₂S or other dopants, is the critical rate-limiting step for Al₂O₃ formation, and to a great extent, controls the minimum temperature at which Al₂O₃ can be deposited. Further it is well established that the water-gas shift reaction is very sensitive for deposition pressure.

Extensive work has been done to deposit CVD Al₂O₃ at lower temperatures. Several Al₂O₃ layers using other than AlCl₃—CO₂—H₂ system have been investigated, including AlCl₃—CO—CO₂, AlCl₃-C₂H₅OH, AlCl₃—N₂O—H₂, AlCl₃—NH₃—CO₂, AlCl₃—O₂—H₂O, AlCl₃—O₂—Ar, AlX₃—CO₂ (where X is Cl, Br, I), AlX₃—CO₂—H₂ (where X is Cl, Br, I), AlBr₃—NO—H₂—N₂ and AlBr₃—NO—H₂—N₂. It is emphasised that these studies have been carried out without dopants (such as H₂S) and the effect of the deposition pressure has not been elucidated.

It is worth noting that none of these systems have been commercially successful. Consequently, to provide a CVD process for depositing Al₂O₃ layers at temperatures below those currently used on a commercial scale is therefore highly desirable.

U.S. Pat. No. 6,572,991 describes a method to deposit γ-Al₂O₃ at low deposition temperatures. This work clearly shows that it is possible to obtain Al₂O₃ layers in the medium temperature range from the AlCl₃—CO₂—H₂ system. However, in this work it was not realised that nucleation surface controls the phase composition of Al₂O₃ and that deposition of α-Al₂O₃ is thus possible at lower deposition temperatures. In the prior-art, it was considered impossible to deposit α-Al₂O₃ at low temperatures and it was believed that γ-Al₂O₃ and κ-Al₂O₃ were the unavoidable low temperature phases.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention is to provide a new, improved alumina layer where the α-Al₂O₃ phase is of nucleated α-Al₂O₃ with a strong, fully controlled (104) growth texture. According to the present invention, α-Al₂O₃ with the controlled (104) texture can be obtained within a wide temperature range from about 750 to about 1000° C., which can be considered surprising.

In one aspect of the invention, there is provided a cutting tool insert of a substrate at least partially coated with a coating with a total thickness of from about 10 to about 40 μm, of one or more refractory layers of which at least one layer is an alumina layer, said alumina layer being composed of columnar α-Al₂O₃ grains with texture coefficients

-   -   a) TC(104) greater than about 2.0;     -   b) TC(012), TC(110), TC(113), TC(024) all less than about 1.0.;     -   c) TC(116) less than about 1.2;

The texture coefficient TC(hkl) is defined as

${{TC}({hkl})} = {\frac{I({hkl})}{I_{O}({hkl})}\left\{ {\frac{1}{n}{\sum\frac{I({hkl})}{I_{O}({hkl})}}} \right\}^{- 1}}$

where

-   -   I(hkl)=measured intensity of the (hkl) reflection     -   I_(o)(hkl)=standard intensity according to JCPDS card no 46-1212     -   n=number of reflections used in the calculation (hkl)         reflections used are: (012), (104), (110), (113), (024), (116).

In another aspect of the invention, there is provided a method of coating a substrate with an Al₂O₃ layer wherein the α-Al₂O₃ layer is composed of columnar α-Al₂O₃ grains with a texture coefficient TC (104) greater than about 2 comprising depositing a (Ti,Al)(C,O,N) bonding layer on said substrate to provide a nucleation surface for said Al₂O₃, subjecting said nucleation surface to a modification treatment of a pulse treatment with a mixture of TiCl₄, AlCl₃ and H₂, a purge with a neutral gas and an oxidizing pulse of a gas mixture including N₂ and CO₂ in a ratio of from about 450 to about 650, repeating the modification treatment and depositing α-Al₂O₃ having a texture coefficient TC(104) greater than about 2 at a temperature of from about 750 to about 1000° C.

The alumina layer with strong texture outperforms the prior art with random or other less developed and incompletely controlled textures. Further, increased toughness can be obtained when deposition is carried out at lower temperatures. Compared with prior-art products the α-Al₂O₃ layer according the present invention is essentially free from transformation stresses, consisting of columnar, defect free, α-Al₂O₃ grains with low dislocation density and with improved cutting properties. The texture-controlled α-Al₂O₃ layers deposited at medium temperature (about 800° C.) show enhanced toughness.

DESCRIPTION OF THE DRAWINGS AND FIGURES

FIG. 1 shows a cross-section SEM image (magnification 5000×) of a typical alumina layer according to the present invention deposited on a MTCVD-Ti(C,N) layer. The alumina layer is composed of columnar grains. It is dense with no detectable porosity.

FIG. 2 shows a cross-section SEM image of a typical layer according the prior-art (magnification 6000×) deposited on a MTCVD-Ti(C,N) layer. The alumina layer is composed of large nearly equiaxed grains. Porosity is visible in the alumina layer. Interfacial porosity between the alumina layer and the Ti(C,N) layer is also visible.

DETAILED DESCRIPTION OF THE INVENTION

A method to deposit α-Al₂O₃ with a strong (104) texture in a temperature range of from about 750 to about 1000° C. is described. The invention utilizes short pulses of precursors followed by purging steps with an inert gas such as Ar. After the purge another precursor is applied as a short pulse. In addition to the texture control, the method can be used to produce finer grain sizes by increasing the number of nucleation sites.

Al₂O₃ layers according to the present invention outperform the prior-art and are especially suitable be used in toughness demanding stainless steel application such as interrupted cutting, turning with coolant and especially intermittent turning with coolant. The other area is cast iron where the edge strength of this kind of alumina layer is superior to the prior art.

Ti(C,N) is used as an intermediate layer, which can be obtained either by conventional CVD or MTCVD, preferably by MTCVD. The present invention makes it possible to deposit α-Al₂O₃ at same temperature as is used to deposit the intermediate MTCVD Ti(C,N) layer. Consequently, the heating-up period can be omitted after MTCVD.

To nucleate α-Al₂O₃ with the specified texture, several steps are needed. First, on the Ti(C,N) layer a bonding layer characterised by the presence of an Al concentration gradient is deposited. Nitrogen and CH₃CN are applied during deposition of this bonding layer. The aluminium content on the surface of this layer is considerably, about 30%, higher than in the bonding layer according to U.S. Pat. No. 5,137,774 (prior-art) and the bonding layer is obviously containing nitrogen. The surface of this bonding layer is subjected to an additional treatment(s).

Nucleation is started with a AlCl₃/TiCl₄/H₂ pulse with a duration of 5 minutes. After that an Ar purge (duration about 5 minutes) is applied in order to remove excess Cl⁻ from the surface. After this, an oxidizing pulse is applied using a CO₂/H₂/N₂/Ar (Co₂=about 0.15%, H₂=about 10%, N₂ about 25%, Ar=balance) gas mixture at a pressure of from about 50 to about 500 mbar, to a temperature of from about 750° to about 1000° C., depending on the temperature of the subsequent alumina deposition. In addition to a relatively low oxidation potential of the gas mixture, the oxidizing step has to relatively short, from about 0.5 to about 5 minutes to secure (104) nucleation. These steps should be repeated several times, preferably from about 2 to about 5 times in sequence to increase the amount of α-Al₂O₃ nuclei. It is noted that if pulsating nucleation is used, one has to find an optimized combination between the duration of the individual steps and the amount of the steps, otherwise too low or excessive oxidization may be obtained. A person skilled in the art can find the correct procedure by trial and error.

The key to obtain the specified growth texture is the control of the oxidation potential of the CO₂/H₂/N₂/Ar mixture by adjustment of the N₂:CO₂ ratio. This ratio should be from about 450 to about 650, preferably from about 450 to about 550. The use of controlled oxygen potential in combination with the correct time and number of pulses enables the correct nucleation mode. Typical pulse times may range from about 10 seconds to about 5 minutes depending on the duration of the pulse. The oxidising pulse is again followed by an Ar purge. These steps should be repeated several times, preferably from about 2 to about 5 times, in sequence to increase the amount of α-Al₂O₃ nuclei. Excessive oxidisation must be avoided. A person skilled in the art can find the best and optimised combination between the duration and the amount of the steps.

Detailed Description of the Nucleation Steps

-   -   1. Depositing a bonding layer from about 0.1 to about 1 μm thick         in a gas mixture of from about 2 to about 3% TiCl₄, AlCl₃         increasing from about 0.5 to about 5%, from about 3 to about 7%         CO, from about 1 to about 3% CO₂, from about 2 to about 10% N₂         and balance H₂ at from about 750 to about 1000° C., preferably         at about 800° C. and at a pressure of from about 50 to about 200         mbar.     -   2. Purging by Ar for about 5 min.     -   3. Treating the bonding layer in a gas mixture of from about 1         to about 2% TiCl₄ and from about 2 to about 4% AlCl₃ in hydrogen         for about 2 to about 60 min at from about 750 to about 1000° C.,         preferably at about 800° C. and at a pressure of from about 50         to about 200 mbar.     -   4. Purging by Ar for 5 about min.     -   5. Treating in a gas mixture of from about 0.1 to about 0.15%         CO₂ (preferably about 0.15%), from about 10 to about 30% N₂         (preferably from about 22.5 to about 30% when the CO₂ content is         about 15%), about 10% H₂, balance Ar at a pressure of from about         50 to about 200 mbar for about 0.5 to about 5 minutes at a         temperature of from about 750 to about 1000° C., depending on         the temperature for the subsequent deposition of the alumina         layer.     -   6. Purging by Ar for about 5 min.     -   7. Repeating steps 3-6 to obtain the an optimum oxidation level.     -   8. Depositing an alumina layer at a temperature of from about         950 to about 1000° C. and a pressure of from about 50 to about         200 mbar with desired thickness according to known technique or         depositing an alumina layer at from 5 about 750 to about 950         using higher deposition pressures (from about 200 to about 500         mbar) together with higher amounts (from about 0.5 to about         1.5%) of catalysing precursors such as H₂S or SO_(x), preferably         H₂S. The growth of the alumina layer onto the nucleation layer         is started by sequencing the reactant gases in the following         order: CO, AlCl₃, CO₂. The process temperatures of from about         750 to about 1000° C. can be used since the texture is         determined by the nucleation surface.

The present invention also relates to a cutting tool insert of a substrate at least partially coated with a coating with a total thickness of from about 15 to about 40 μm, preferably from about 20 to about 25 μm, of one or more refractory layers of which at least one layer is an alpha alumina layer. The α-Al₂O₃ layer deposited according to the present invention is dense and exhibits a very low defect density. It is composed of columnar grains with a strong (104) texture. The columnar grains have a length/width ratio of from about 2 to about 12, preferably from about 4 to about 8.

The texture coefficients (TC) for the α-Al₂O₃ according to the present invention layer is determined as follows:

${{TC}({hkl})} = {\frac{I({hkl})}{I_{O}({hkl})}\left\{ {\frac{1}{n}{\sum\frac{I({hkl})}{I_{O}({hkl})}}} \right\}^{- 1}}$

where

I(hkl)=intensity of the (hkl) reflection

I_(o)(hkl)=standard intensity according to JCPDS card no 46-1212

-   -   n=number of reflections used in the calculation (hkl)         reflections used are: (012), (104), (110), (113), (024), (116).     -   The texture of the alumina layer is defined as follows:     -   TC(104) greater than about 2.0, preferably greater than about 3.         Simultaneously TC(012), TC(110), TC(113), TC(024) should be all         less than about 1.0, preferably less than about 0.5. Note that         the related (012) and (024) reflections are also low. However,         for this growth mode TC(116) is somewhat higher than the other         background reflections. TC(116) should be less than about 1.2,         preferably less than about 1.

The substrate comprises a hard material such as cemented carbide, cermets, ceramics, high speed steel or a super hard material such as cubic boron nitride (CBN) or diamond, preferably cemented carbide or CBN. With CBN is herein meant a cutting tool material containing at least about 40 vol-% CBN. In a preferred embodiment, the substrate is a cemented carbide with a binder phase enriched surface zone.

The coating comprises a first layer adjacent the body of CVD Ti(C,N), CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVD Ti(B,C,N), CVD HfN or combinations thereof preferably of Ti(C,N) having a thickness of from about 1 to about 20 μm, preferably from about 1 to about 10 μm, and said α-Al₂O₃ layer adjacent said first layer having a thickness of from about 1 to 40 μm, preferably from about 1 to about 20 μm, most preferably from about 1 to about 10 μm. Preferably, there is an intermediate layer of TiN between the substrate and said first layer with a thickness of less than about 3 μm, preferably from about 0.5 to about 2 μm.

In one embodiment, the α-Al₂O₃ layer is the uppermost layer.

In another embodiment, there is a layer of carbide, nitride, carbonitride or carboxynitride of one or more of Ti, Zr and Hf, having a thickness of from about 0.5 to about 3 μm, preferably from about 0.5 to about 1.5 μm, atop the α-Al₂O₃ layer. Alternatively this layer has a thickness of from about 1 to about 20 μm, preferably from about 2 to about 8 μm.

In yet another embodiment, the coating includes a layer of κ-Al₂O₃ and/or γ-Al₂O₃ preferably atop the α-Al₂O₃. with a thickness of from 0.5 to 10, preferably from 1 to 5 μm.

The invention is additionally illustrated in connection with the following examples, which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the examples.

EXAMPLE 1

Cemented carbide cutting inserts with a composition of 5.9% Co and balance WC (hardness about 1600 HV) were coated with a layer of MTCVD Ti(C,N). The thickness of the MTCVD layer was about 2 μm. On to this layer two different layers consisting of about 10 μm α-Al₂O₃ were deposited:

Layer a) contained a (104) textured layer and was deposited according to the present invention at 1000° C. The detailed process data is given in Table 1.

Layer b) was deposited according to the prior art.

Layer c) contained a (104) textured layer and was deposited according to the present invention at 800° C. The detailed process data is given in Table 2.

TABLE 1 Deposition process for a Layer a) with (104) texture at 1000° C.: Step 1: Bonding layer Gas mixture TiCl₄ = 2.8% AlCl₃ = 0.8-4.2% CO = 5.8% CO₂ = 2.2% N₂ = 5-6% Balance: H₂ Duration 60 min Temperature 1000° C. Pressure 100 mbar Step 2: Purge Gas Ar = 100% Duration 5 min Temperature 1000 C. Pressure 50 mbar Step 3: Pulse 1 Gas mixture TiCl₄ = 1.6% AlCl₃ = 2.8 H₂ = Balance Duration 2-5 min depending on the amount of pulses. Temperature 1000 C. Pressure 50 mbar Step 4: Purge Gas Ar = 100% Duration 5 min Temperature 1000 C. Pressure 50 mbar Step 5: Pulse 2 Gas mixture CO₂ = 0.05% N₂ = 25% Balance: H₂ Duration 0.5-1 min depending on the amount of pulses. Temperature 1000° C. Pressure 100 mbar Step 6: Purge Gas Ar = 100% Duration 5 min Temperature 1000 C. Pressure 50 mbar Step 7: Nucleation step Gas mixture AlCl₃ = 3.2% HCl = 2.0% CO₂ = 1.9% Balance H₂ Duration 60 min Temperature 1000° C. Pressure 210 mbar Step 8: Deposition Gas mixture AlCl₃ = 4.2% HCl = 1.0% CO₂ = 2.1% H₂S = 0.2% Balance: H₂ Duration 520 min Temperature 1000° C. Pressure 50 mbar

It should be noted that the steps 3-7 can be repeated 5-10 times in sequence in order to obtain grain refinement and the strong desired texture. The amount of pulses can be even higher if the duration of step 6 is reduced. In this example the steps 3-7 were repeated 3 times with durations of 0.6 minutes.

TABLE 2 Deposition process for a Layer c) with (104) texture at 780° C.: Step 1: Bonding layer Gas mixture TiCl₄ = 2.8% CH₃CN = 0.7% AlCl₃ = increasing from 0.8 to 4.2% CO = 5.8% CO₂ = 2.2% N₂ = 5% Balance: H₂ Duration 40 min Temperature 780° C. Pressure 100 mbar Step 2: Purge Gas Ar = 100% Duration 5 min Temperature 780° C. Pressure 50 mbar Step 3: Pulse 1 Gas mixture TiCl₄ = 1.6% AlCl₃ = 2.8 H₂ = Balance Duration 5 min. Temperature 780° C. Pressure 50 mbar Step 4: Purge Gas Ar = 100% Duration 5 min Temperature 780° C. Pressure 50 mbar Step 5: Pulse 2 Gas mixture CO₂ = 0.05% N₂ = 25% H₂ = 10% Balance: Ar Duration 2 min Temperature 780° C. Pressure 100 mbar Step 6: Purge Gas Ar = 100% Duration 5 min Temperature 780° C. Pressure 50 mbar Step 7: Nucleation step Gas mixture AlCl₃ = 3.2% HCl = 2.0% CO₂ = 1.9% Balance H₂ Duration 60 min Temperature 780° C. Pressure 50 mbar Step 8: Deposition Gas mixture AlCl₃ = 4.1% HCl = 1.0% CO₂ = 2.3% H₂S = 0.9% Balance: H₂ Duration 600 min Temperature 780° C. Pressure 350 mbar Steps 3-6 were repeated three times.

EXAMPLE 2

Layers a, b and c were studied using X-ray diffraction. The texture coefficients were determined are presented in Table 3. As clear from Table 3 the layer according to the present invention exhibits a strong (104) texture when deposited either at 1000° C. or 780° C. Typically, for this growth mode the (116) reflection is somewhat more profound than the other background reflections.

TABLE 3 Hkl Invention, layer a Prior art, layer b, Invention, layer c 012 0.24 0.97 0.49 104 4.30 1.14 3.13 110 0.06 0.95 0.49 113 0.19 0.99 0.41 024 0.27 0.86 0.49 116 0.94 1.09 0.99

EXAMPLE 3

Layers a) and b) were studied using Scanning Electron Microscopy. The cross section images of the layers are shown in FIGS. 1 and 2, respectively. The differences in microstructure and morphology are clear.

EXAMPLE 4

The layers a) and b) from the Example 1 were tested with respect to edge chipping in longitudinal turning of cast iron.

Work piece: Cylindrical bar Material: SS0130 Insert type: SNUN Cutting speed: 400 m/min Feed: 0.4 mm/rev Depth of cut: 2.5 mm Remarks: dry turning

The inserts were inspected after 2 and 4 minutes of cutting, As clear from Table 4 the edge toughness of the prior art product was considerably enhanced when the layer was produced according to the present invention.

TABLE 4 Flaking of the edge Flaking of the edge line (%) after 2 minutes line (%) After 6 minutes Layer a (Invention) 0 6 Layer b 12 22

EXAMPLE 5

The layer produced according to the present invention was compared with a market leader, referred here as Competitor X. This coating is composed of MTCVD Ti(C,N) and α-Al₂O₃. XRD was used to determine the texture coefficients for these competitor coatings. Two inserts from Competitor X were randomly chosen for XRD. Table 5 shows the obtained TCs for the Competitor X. The coatings from Competitor X exhibit a random texture and can be compared with the present invention, Table 1.

TABLE 5 Hkl TC(hkl) 012 0.71 0.57 104 0.92 0.86 110 1.69 1.92 113 0.48 0.40 024 1.16 1.14 116 1.04 1.11 The X-rayed inserts from the competitor X were compared with inserts produced according to the present invention, Layer a).

Two inserts produced according to the present invention were compared with the two Competitor X inserts with respect to flank wear resistance in face turning of ball bearing material:

Work piece: Cylindrical tubes (Ball bearings) Material: SS2258 Insert type: WNMG080416 Cutting speed: 500 m/min Feed: 0.5 mm/rev Depth of cut: 1.0 mm Remarks: Dry turning Tool life criterion: Flank wear >0.3 mm, three edges of each variant were tested.

Results: Tool life (min) Layer a 25.0 (invention) Layer a 23.5 (invention) Competitor 1 14.5 (prior art) Competitor 2 15.5 (prior art)

EXAMPLE 6

Layer a), b) and c) deposited on Co-enriched substrates were tested with respect to toughness in longitudinal turning with interrupted cuts.

Work piece: Cylindrical slotted bar Material: SS1672 Insert type: CNMG120408-M3 Cutting speed: 140 m/min Feed: 0.1, 0.125, 0.16, 0.20, 0.25, 0.315, 0.4, 0.5, 0.63, 0.8 mm/rev gradually increased after 10 mm length of cut Depth of cut: 2.5 mm Remarks: dry turning Tool life criteria: Gradually increased feed until edge breakage. 10 edges of each variant were tested.

TABLE 6 Mean feed at breakage (mm/rev) Layer a (invention) 0.24 Layer b (prior art) 0.12 Layer c (invention) 0.32

The test results show (Table 6) that layers according to the present invention exhibited clearly better toughness behaviour than the prior-art (layer b).

EXAMPLE 7

Cubic boron nitride (CBN) insert containing about 90% of polycrystalline CBN (PCBN) were coated according to the present invention and according to prior art layer discussed in Example 1. The coated CBN was compared with uncoated CBN insert in cutting of steel containing ferrite. It is known that B has a high affinity to ferrite and diffusion wear occurs at high cutting speeds. As shown in Table 7 the layer according to the present invention is superior to the prior art.

Work piece: Cylindrical bar Material: SS0130 Insert type: SNUN Cutting speed: 750 m/min Feed: 0.4 mm/rev Depth of cut: 2.5 mm Remarks: dry turning

TABLE 7 Life time (min) Coated CBN (Invention) 26 Coated according to prior art 11 Uncoated CBN 9

Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims. 

1. A method of coating a substrate with an Al₂O₃ layer wherein the α-Al₂O₃ layer is composed of columnar α-Al₂O₃ grains with a texture coefficient TC (104) greater than about 2 comprising depositing a (Ti,Al)(C,O,N) bonding layer on said substrate to provide a nucleation surface for said Al₂O₃, subjecting said nucleation surface to a modification treatment of a pulse treatment with a mixture of TiCl₄, AlCl₃ and H₂, a purge with a neutral gas and an oxidizing pulse of a gas mixture including N₂ and CO₂ in a ratio of from about 450 to about 650, repeating the modification treatment and depositing α-Al₂O₃ having a texture coefficient TC(104) greater than about 2 at a temperature of from about 750 to about 1000° C.
 2. The method of claim 1 wherein each said oxidizing pulse treatment is conducted for a time of from about 0.5 to about 5 minutes.
 3. The method of claim 1 wherein the neutral gas is argon.
 4. The method of claim 1 wherein the mixture of TiCl₄, AlCl₃ and H₂ comprises a mixture of about 1 to about 3% TiCl₄ from about 2 to about 4% AlCl₃, balance H₂.
 5. The mixture of claim 4 wherein the said pulse treatment with TiCl₄, Al₂O₃ and H₂ is conducted for a time of from about 2 to about 60 minutes at a temperature of from about 750 to 1000° C. and a pressure of from about 50 to about 200 mbar.
 6. The method of claim 1 wherein the oxidizing pulse comprises a mixture of from about 0.05 to about 0.1% CO₂ from about 20 to about 65% N₂, about 10% H₂, balance Ar.
 7. The method of claim 6 wherein the oxidizing pulse is conducted for a time of from about 0.5 to about 5 minutes, a temperature of from about 750° to about 1000° C. and a pressure of from about 50 to about 500 mbar.
 8. The method of claim 1 wherein the α-Al₂O₃ deposition is conducted at a temperature of from about 950 to about 1000° C.
 9. The method of claim 1 wherein the α-Al₂O₃ deposition is conducted at a temperature of from about 750 to 950° C. at a pressure of from about 200 to about 500 mbar.
 10. The method of claim 9 wherein there is present a catalyzing precursor in an amount of from about 0.5 to about 1.5%.
 11. The method of claim 10 wherein the catalyzing precursor is H₂S or SO_(X).
 12. The method of claim 11 wherein the catalyzing precursor is H₂S. 